Light emitting flat panel devices are used for a number of applications such as general illumination light sources, decorative light sources, and information displays. Organic light emitting diode (OLED) devices are attractive for use in flat panel devices because of their low driving voltage, high luminance, wide-angle viewing, and capability for full-color flat emission displays. Tang et al., e.g., described a multilayer OLED device in their U.S. Pat. Nos. 4,769,292 and 4,885,211. There are many potential applications in which changing the color depending on the angle of the viewer with respect to a flat panel device may provide added value or information, especially if the flat panel device were to provide both angular dependent as well as non-angular dependent patterned emissions. For example, aircraft approaching a runway need to determine whether they are on, above, or below the glide-path. For aircraft operating under visual flight rules, the relationship to the glide-path is currently determined by various means which provide an angular measurement with respect to the face of the device. A flat panel light source with patterned color emissions dependent on view angle used as a glide-path indicator could provide a greater wealth of information. In architectural applications, a view angle dependent color emission might be used either to convey information about location and relative position, or it may be used for a purely decorative function. It is well-known that all thin-film devices demonstrate some degree of angular dependence in their emissions, but these variations are typically so small as to be unnoticed except by careful observation or the use of sensitive instruments.
Efficient broad-band, or white light producing OLED devices are considered low cost alternatives for several applications such as paper-thin light sources, backlights in LCD displays, automotive dome lights, and office lighting. White light producing OLED devices should be bright, efficient, and generally have a broad emission spectrum that covers most of the visible wavelength range. Herein the term “white” or “substantially white” is used broadly to mean light that is perceived as white or off-white. “Broad-band” is used to include white light, as well as other emission spectrums with multiple emission spectrum peaks or with single peaks having a relatively broad range (e.g., half peak bandwidths preferably of 100 nm or more).
White light producing OLED devices have been reported before by J. Shi in U.S. Pat. No. 5,683,823 wherein the luminescent layer includes red and blue light emitting materials uniformly dispersed in a host emitting material. This device has good electroluminescent characteristics, but the concentrations of the red and blue dopants are very small, such as 0.12% and 0.25% of the host material. These concentrations are difficult to control during large-scale manufacturing. Sato et al. in JP 07,142,169 discloses an OLED device capable of emitting white light, made by forming a blue light emitting layer next to the hole-transporting layer and followed by a green light emitting layer having a region containing a red fluorescent layer.
Kido et al., in Science, Vol. 267, p. 1332 (1995) and in Applied Physics Letters, Vol. 64, p. 815 (1994), report a white light producing OLED device. In this device, three emitter layers with different carrier transport properties, each emitting blue, green, or red light, are used to generate white light. Littman et al. in U.S. Pat. No. 5,405,709 disclose another white emitting device, which is capable of emitting white light in response to hole-electron recombination, and comprises a fluorescent in a visible light range from bluish green to red. Deshpande et al., in Applied Physics Letters, Vol. 75, p. 888 (1999), describe a white OLED device using red, blue, and green luminescent layers separated by a hole blocking layer.
OLED devices typically have at least one transparent electrode, which is often fabricated from a conductive oxide such as indium-tin oxide (ITO). Such materials have sufficient conductivity for displays, in which the individual pixels are on the order of 1 mm or less. However, the conductivity of such transparent electrodes can be insufficient for applications with much larger individual emitting units, such as flat panel lighting. This disadvantage can be overcome by making the emitting elements in narrow stripes, but registration difficulties would make such a device, more difficult to fabricate, increasing the manufacturing cost. Furthermore, such transparent electrodes are themselves costly and add to the manufacturing cost.
Co-pending, commonly assigned U.S. patent application Ser. No. 10/680,758 by Yuan-Sheng Tyan et al., the disclosure of which is incorporated herein by reference, describes a white OLED apparatus with high illumination efficiency comprising a microcavity OLED device and a light-integrating element, wherein the microcavity OLED device has a white light emitting organic EL element and the microcavity OLED device is configured to have angular-dependent narrow-band emission, and the light-integrating element integrates the angular-dependent narrow-band emission from different angles from the microcavity OLED device to form white light emission. Such white light emitting OLED apparatus which utilizes an OLED device having only metal electrodes enables reduced cost, higher conductivity, and improved ease of manufacturing.